There has been known the technology in which a circuit for supplying power to the motor of an OA equipment or a FA equipment is formed as an integrated circuit (IC) (See for example Japanese Patent Laid-Open Publication No. SHO 63-171170). There are strong needs in recent years for downsizing the IC output circuit as well as to realize a high rotational speed for the motor. To achieve this goal, the IC is required to withstand high voltage and large current. To enable use of a large current in an IC, it is effective to use a pulse width modulating system (described as PWM hereinafter) for a control system for an output transistor. The power supplied to the IC can be reduced with the PWM system.
To provide controls for driving a multi-phase motor using the PWM system (described as PWM control) with an IC output circuit based on a bipolar transistor system, a current path using a diode is provided in order to discharge the energy stored in the coil of the motor. A current path returning from the motor to a power supply unit is provided by connecting an anode of the diode to an output terminal of the IC output circuit and connecting a cathode to the power supply. A current path from the ground (GND) is provided by connecting the anode of the diode to the ground GND and connecting the cathode to the output terminal of the IC output circuit. Each of the diodes requires the same capability for allowing flow of current therethrough as that of the output transistor, so that they require a junction area of the same order to that of the output transistor.
FIG. 5 is a circuit diagram showing configuration of a conventional type of IC output circuit based on the pulse width modulating system, and FIG. 6 is a timing chart showing timing of operations of the IC output circuit. This IC output circuit has, for instance, a triangular wave generating circuit 102, a comparator 103, a position detecting circuit 104, an output element control circuit 105, a capacitor C1, six NPN transistors 111, 112, 113, 114, 115 and 116 and six diodes 123, 124, 125, 126, 127 and 128.
The IC output circuit is connected to three output terminals OUT1, OUT2, and OUT3 of a three-phase spindle motor M, a speed control circuit 101 connected to the outside, three Hall elements H1, H2 and H3, and two resistors R1, R2. In FIG. 5, a region enclosed with a dot and dash line indicates an IC chip, and the white dots indicate terminals.
Each NPN transistor 111, 112, 113, 114, 115 and 116 is a switching element for output, and the base of each of them is connected via signal lines B1, B2, B3, B4, B5 and B6 respectively to the output element control circuit 105. The collector of the first NPN transistor 111 is connected to a power terminal VCC as well as to the cathode of the first diode 123 and the emitter is connected to the first output terminal OUT1 as well as to the anode of the first diode 123.
The collector of the second NPN transistor 112 is connected to the power terminal VCC as well as to the cathode of the second diode 124 and the emitter is connected to the second output terminal OUT2 as well as to the anode of the second diode 124.
The collector of the third NPN transistor 113 is connected to the power terminal VCC as well as to the cathode of the third diode 125 and the emitter is connected to the output terminal OUT3 as well as to the anode of the third diode 125.
The collector of the fourth NPN transistor 114 is connected to the emitter of the first NPN transistor 111 as well as to the cathode of the fourth diode 126, and the emitter of the fourth NPN transistor 114 is connected to the ground GND as well as to the anode of the fourth diode 126.
The collector of the fifth NPN transistor 115 is connected to the emitter of the second NPN transistor 112 as well as to the cathode of the fifth diode 127, and the emitter of the fifth NPN transistor 115 is connected to the ground GND as well as to the anode of the fifth diode 127.
The collector of the sixth NPN transistor 116 is connected to the emitter of the third NPN transistor 113 as well as to the cathode of the sixth diode 128, and the emitter of the sixth NPN transistor 116 is connected to the ground GND as well as to the anode of the sixth diode 128.
The speed control circuit 101 generates a control reference voltage signal S10. The triangular wave generating circuit 102 charges or discharges the capacitor C1 with a constant current, and thereby a triangular wave signal S11 is generated in the upper side of the capacitor. When the control reference voltage signal S10 is inputted via the reference voltage terminal VCTL into the comparator 103, the comparator 103 compares the control reference voltage signal S10 to that of the triangular wave signal S11.
When the potential of the control reference voltage signal S10 is higher than that of the triangular wave signal S11, the potential of an output signal S12 from the comparator is set in a relatively high "H" level. While the signal S12 is at the "H" level, PWM control is ON (Refer to the period "A" in FIG. 6). When the potential of the control reference voltage signal S10 is lower than that of the triangular wave signal S11, the potential of the output signal S12 from the comparator 103 is set in a relatively low "L" level, while PWM control is kept OFF (Refer to the period "B" in FIG. 6).
The Hall elements H1, H2 and H3 output signals S1, S2, S3, S4, S5 and S6 each indicating a position of the motor M. The position detecting circuit 104 outputs output control signals S7, S8 and S9 for deciding a phase of a current flow in the motor M according to the position signals S1, S2, S3, S4, S5 and S6. The resistors R1 and R2 limit the currents flowing through the Hall elements H1, H2 and H3.
The output element control circuit 105 supplies a current into any of the signal lines B1, B2 and B3 to turn ON any of the upper-side NPN transistors 111, 112 and 113 according to the control signals S7, S8 and S9 outputted from the position detecting circuit 104 when the output signal S12 from the comparator 103 is at the "H" level.
However, the first NPN transistor 111 and the fourth NPN transistor 114 are never turned ON simultaneously. Similarly the second NPN transistor 112 and the fifth NPN transistor 115 are never turned ON simultaneously, and further the third NPN transistor 113 and the sixth NPN transistor 116 are not turned ON simultaneously.
In other words, NPN transistors in the same phase are not turned ON simultaneously. On the other hand, the output element control circuit 105 does not supply a current to any of the six signal lines B1, B2, B3, B4, B5 and B6 while the output signal S12 from the comparator 103 is at the "L" level. Therefore, a diode is turned ON that has the same phase as the phase of the transistor which is ON when the signal S12 is at "H" level and whose upper side (the side towards VCC) and lower side (the side towards GND) are connected oppositely as explained in the next paragraph.
For instance, when the first NPN transistor 111 (having upper side connected to VCC) is ON, the fourth diode 126 (having the same phase and having the upper side connected to the GND) is turned ON, and when the second NPN transistor 112 or the third NPN transistor 113 (having upper side connected to VCC) is ON, the fifth diode 127 or the sixth diode 128 (having the same phase and having the upper side connected to the GND) is respectively turned ON. Similarly, when the fourth NPN transistor 114, fifth NPN transistor 115 or the sixth NPN transistor 116 is ON, the first diode 123, second diode 124 or the third diode 125 is respectively turned ON.
Next, operations of the IC output circuit shown in FIG. 5 are described. To describe a case when PWM control is ON, namely when, for instance, the first NPN transistor 111 and the fifth NPN transistor 115 are turned ON, output current flows from the power terminal VCC via the first NPN transistor 111 to the first output terminal OUT1 to the motor M to the second output terminal OUT2 to the fifth NPN transistor 115 and finally to the ground GND.
When the PWM control is switched OFF, both the first NPN transistor 111 and the fifth NPN transistor 115 are turned OFF, while the fourth diode 126 and the second diode 124 are turned ON due to discharge of energy stored in the motor M. Therefore, a current flows from the ground GND via the fourth diode 126 to the first output terminal OUT1 to the motor M to the second output terminal OUT2 to the second diode 124 and finally to the power terminal VCC.
When the PWM control is again switched ON, a current flows from any of the first to third NPN transistors 111, 112 and 113 to any of the fourth to sixth NPN transistors 114, 115 and 116.
As described above, when PWM control is OFF, the discharged current flows through any of the two diodes of the six diodes 123, 124, 125, 126, 127 and 128. This current is the energy stored in the coil of the motor M and its current quantity is substantially same as that of an output current from the motor M.
Thus, each of the diodes 123, 124, 125, 126, 127 and 128 requires a capability of allowing a current to flow. Therefore, when the IC output circuit shown in FIG. 5 is formed in an IC by means of bipolar transistor processing, as shown in the pattern layout circuit of FIG. 7, the percentage of the area occupied by the diodes 123, 124, 125, 126, 127 and 128 becomes very large.
In FIG. 7, regions 102, 103, 104 and 105 represent the places where the triangular generating circuit 102, comparator 103, position detecting circuit 104 and output element control circuit 105 are provided. Regions 111, 112, 113, 114, 115 and 116 represent the places where the NPN transistors 111, 112, 114, 114, 115 and 116 are provided. Finally, regions 123, 124, 125, 126, 127 and 128 represent the places where the diodes 123, 124, 125, 126, 127 and 128 are provided.
When a current path returned from the ground GND to the power terminal VCC is to be established, the current path is established in the upper side by turning OFF the fourth to sixth NPN transistors 114, 115 and 116, or in the lower side by turning OFF the first to third NPN transistors 111, 112 and 113.
As described above, when a conventional type of IC output circuit is made by means of bipolar transistor processing, the area occupied by the diodes which provide a current path for the discharged current becomes larger when PWM control is OFF, so that it is difficult to reduce the area of the pattern layout, which, in turn, makes it difficult to down-size the IC chip.
Further, when PWM control is OFF, potentials at the output terminals OUT1, OUT2 and OUT3 become lower than the ground potential GND, so that a parasitic transistor may be formed. To prevent this phenomenon, it is desirable to form the diodes 126, 127 and 128 with a Schottky diode having a low regular-directional voltage, but a Schottky diode is expensive, so that it is difficult to reduce the IC cost.